1. Field of the Invention
The present invention relates to a TTL auto focus controlling system, and is particularly concerned with an auto focus controlling system operating for focus control as computing new focused positions successively according to accumulating operation of a charge storage sensor while driving a camera lens to focused positions.
2. Related Background Art
Known hitherto is such type of the auto focus controlling system wherein a camera lens is driven by motor. A luminous flux, from an object to be photographed is passed through the camera lens and is imaged on a light receiving element such as a CCD which is provided within a camera body to detect the amount of defocus, and focusing is then controlled through the focused state.
Generally, such auto focus controlling system operates for "range finding" to detect a focused state of optical system, and "lens drive" to drive a camera lens according to a result of range finding. The case where the focusing operation is complete in one cycle of "range finding" and "lens drive" may involve no problem. However, if the amount of defocus becomes appreciably large, the range finding becomes short of precision, as that "range finding" and "lens drive" must be repeated after the lens is driven once. In case the object moves continuously, "range finding" and "lens drive" must also be repeated so as to keep focusing dynamically after the subject.
In such case, a response efficiency of the system may be enhanced from overlapping range finding and lens drive in time. However, in the auto focus controlling system using a charge storage sensor such as CCD or the like as a light receiving element, a storage time according to an intensity of light is required for converting an optical image on the sensor into photoelectricity and thus obtaining a video signal having a proper S/N ratio. If a range finding is carried out while a camera lens is driven, a light intensity distribution on the sensor changes during the storage time due to a drive of the camera lens. Therefore it cannot be estimated with precision whether or not the amount of defocus obtainable from processing a sensor output after the storage is over through a predetermined time is equivalent to a particular lens position through which the camera lens is driven.
Thus, as the sequence for servoing the camera lens for the auto focus control, a cycle wherein the range finding is done as keeping the camera lens still, and the camera lens is then driven according to a result of range finding is repeated as mentioned hereinbefore. Or, in case the lens drive is effected during the sensor storage period, the obtained amount of defocus is corrected on the assumption that the camera lens moves at uniform velocity at least during the sensor storage period, and that the obtained amount of defocus thus corresponds to a camera lens position midway between the lens positions at the start and end of the storage period. The next servo drive is then carried out.
With the axis of ordinate as indicating a lens position Z and the axis of abscissa as indicating a time t, FIG. 5 represents a prior art method wherein a range finding and lens drive are repeated alternately in drive and stop fashion in an autofocus servo system, showing a mode wherein a camera lens is driven to a focused position (Z=0). In the drawing, the sensor is accumulating at times indicated by oblique lines for computing the amount of defocus. FIG. 6 is a flowchart indicating a sequence wherein range finding and lens drive of FIG. 5 are repeated alternately. FIG. 7 is an explanatory drawing of a lens drive when range finding and lens drive are overlapped and a moving rate of the camera lens during accumulation of the sensor is assumed constant.
As illustrated in FIG. 7, if the lens moving rate is constant during sensor storage times (times for range finding) T1, T2, T3, then range finding positions corresponding to a range finding result (amount of defocus) obtained through the storage times T1, T2, T3 are P1, P2, P3 respectively, these being equivalent to mid-points of the storage times T1, T2, T3.
The FIG. 7 scheme is problematic as a practical matter, because the assumption of a constant lens moving rate often does not hold. For example, the rate of lens movement will change when a driving motor is started and braked, or a driving rate is modified. Moreover, even when the lens is subjected to a steady drive, if a mechanical design and a fabrication are not satisfactory, then a motor load varies considerably according to a position of the lens, and thus a constant rate of the motor cannot be so assumed in most cases. Therefore, a lens move corresponding to the amount of defocus cannot be corrected so precisely as might be expected.
FIG. 8 represents the case where a lens driven speed is not constant during the period of time for accumulating operation of the sensor (operation for range finding), again indicated by oblique lines. As will be appreciated from the preceding discussion of FIG. 7, the method for correcting the amount of defocus on the assumption that the lens driven speed is constant is not satisfactory in the case represented by FIG. 8.